An injection (often referred to as a "shot" in US English, or a "jab" in UK English) is an infusion method of putting fluid into the body, usually with a syringe and a hollow needle which is pierced through the skin to a sufficient depth for the material to be administered into the body. An injection follows a parenteral route of administration; that is, administration via a route other than through the digestive tract. Since the process inherently involves a small puncture wound to the body (with varying degrees of pain depending on injection type and location, medication type, needle gauge and the skill of the individual administering the injection), fear of needles is a common phobia.

Injections are among the most common health care procedures, with at least 16 billion administered in developing and transitional countries each year. 95% of injections are administered in curative care, 3% are for immunization, and the rest for other purposes, such as blood transfusions.[1] In some instances the term injection is used synonymously with inoculation even by different workers in the same hospital. This should not cause confusion; the focus is on what is being injected/inoculated, not the terminology of the procedure.

In an intramuscular injection, the medication is delivered directly into a muscle. Many vaccines are administered intramuscularly, as are codeine, metoclopramide, and many other medications. Many drugs injected intramuscularly are absorbed into the muscle fairly quickly, while others are more gradual. Injections to the buttocks are known to reach the bloodstream quickly due to the large amount of muscular tissue and corresponding blood supply.

Generally, intramuscular injections are administered by a trained medical professional; however, prescribed self-administered intramuscular injections are becoming more common for patients who require these injections routinely.

A depot injection is an injection, usually subcutaneous or intramuscular, of a pharmacological agent which releases its active compound in a consistent way over a long period of time. Depot injections are usually either solid or oil-based. Depot injections may be available as certain forms of a drug, such as decanoate salts or esters. Examples of depot injections include Depo Provera and haloperidol decanoate. Prostate cancer patients receiving hormone therapy usually get depot injections as a treatment or therapy. Zoladex is an example of a medication delivered by depot for prostate cancer treatment or therapy.

The advantages of using a long-acting depot injection include increased medication compliance due to reduction in the frequency of dosing, as well as more consistent serum concentrations. A significant disadvantage is that the drug is not immediately reversible, since it is slowly released. In psychiatric nursing, a short acting depot, zuclopenthixol acetate (Clopixol Acuphase), which lasts in the system from 24 – 72 hours, is now more regularly used for rapid tranquillisation.[2]

The pain of an injection may be lessened by prior application of ice or topical anesthetic, or simultaneous pinching of the skin. Recent studies suggest that forced coughing during an injection stimulates a transient rise in blood pressure which inhibits the perception of pain.[3] Sometimes, as with an amniocentesis, a local anesthetic is given.[4] The most common technique to reduce the pain of an injection is simply to distract the patient.

Babies can be distracted by giving them a small amount of sweet liquid, such as sugar solution, during the injection, which reduces crying.[5]

40% of injections worldwide are administered with unsterilized, reused syringes and needles, and in some countries this proportion is 70%, exposing millions of people to infections.

Another risk is poor collection and disposal of dirty injection equipment, which exposes healthcare workers and the community to the risk of needle stick injuries. In some countries, unsafe disposal can lead to re-sale of used equipment on the black market. Many countries have legislation or policies that mandate that healthcare professionals use a safety syringe (safety engineered needle) or alternative methods of administering medicines whenever possible.

A needle tract infection is an infection that occurs when pathogenicmicro-organisms are seeded into the tissues of the body during an injection.[8] Such infections are also referred to as needlestick infections.